Techniques for acquiring images by full-field optical-coherence-tomography (OCT) imaging with incoherent light are described for interference-microscopy applications and are very effective non-invasive and non-destructive methods for acquiring images at depth in biological tissues.
A full-field interference-microscopy technique is for example described in the article “Full-field optical coherence tomography” by A. Dubois a C. Boccara, extracted from the work “Optical Coherence Tomography—Technology and Applications”—Wolfgang Drexler—James G. Fujimoto—Editors—Springer 2009, the experimental setup of which is reproduced in FIG. 1A.
The interference-microscopy system 10 known from the prior art and reproduced in FIG. 1A in particular comprises an illumination channel 1 with a low-coherence light source 11, and a detection channel 2 with a camera 21 and a lens 22 (so-called tube lens) allowing a plane of the sample S that it is desired to image and a detection plane of the camera to be optically conjugated. The system 10 moreover comprises an interferometer 3—in this example a Linnik device—comprising a splitting element 30 suitable for receiving the light waves emitted by the source 11 and for sending them to an object arm in which the sample S is arranged and a reference arm in which a reference mirror 34 is arranged, respectively. In each of the object and reference arms are positioned identical microscope objectives, referenced 31 and 32 in FIG. 1A, respectively. Axial moving devices 33, 35, for example piezoelectric motors, allow the sample S and the reference mirror 34 to be moved along the optical axis of the microscope objective 31 and along the optical axis of the microscope objective 32, respectively. In practice, the sample S, for example a sample of biological tissue, may be applied against a silica window that ensures the planarity of its surface and thus avoids aberrations that topographical irregularities in the surface of the sample could induce. To decrease the parasitic signal of reflection from the surface of the window, an antireflection treatment may be deposited thereon or an immersion objective with a coupling liquid that possesses a refractive index similar to that of silica may be used.
The full-field OCT imaging technique is based on the exploitation of light backscattered by the sample when it is illuminated by a light source of small coherence length, and in particular the exploitation of light backscattered by cellular and tissular microscopic structures in the case of a biological sample. This technique exploits the low temporal coherence of the light source to isolate the light backscattered by a virtual slice located at depth in the sample. The use of an interferometer allows an interference signal representative of the light selectively originating from a given slice of the sample to be generated, via interference between the light waves backscattered by the sample and the light waves reflected by the reference mirror, and light originating from the rest of the sample to be eliminated. This technique allows three-dimensional images with a typical resolution of about 1 μm to be obtained.
Very recently, E. Auksorius et al. (“Dark-field full-field optical coherence tomography”, Optics Letters, Vol. 40, No. 14 (2015)) have shown how the contrast of the images may be clearly improved, in a full-field interference-microscopy system of the type shown in FIG. 1A, by virtue of suppression of the specular reflections from the air/glass interfaces formed by the windows of the elements for holding the sample in particular.
FIG. 1B shows a schematic of the experimental setup described in the article by E. Auksorius et al. cited above. The interference-microscopy system 20 shown in FIG. 1B comprises, just like that shown in FIG. 1A, an illumination channel 1, a detection channel 2 and an interferometer 3. The interferometer 3 comprises, in each of the object and reference arms formed by means of the splitting element 30, a microscope objective, referenced 31 and 32, respectively. The illumination channel comprises, in addition to the light source 11, an optical system 12, 13 for imaging the source 11 in an image focal plane of the microscope objectives. The image focal planes of the microscope objectives 31, 32 are coincident with the pupillary planes P1, P2 of the microscope objectives 31 and 32, respectively. An aperture diaphragm arranged in a plane 14 that is substantially coincident with that of the source 11 allows the spatial extent of the source to be adjusted and a field diaphragm arranged in a plane 15 that is conjugate with the reference mirror allows the field of view on the sample to be adjusted. The detection channel 2 comprises, in addition to the camera 21 and to the tube lens 22, an optical relay system 23, 24, allowing a pupillary plane P that is conjugate with the pupillary planes P1, P2 to be formed. The suppression of the specular reflections is obtained by means of an opaque disc 25 positioned in the detection channel, in the conjugate plane P of the pupillary planes P1, P2. Moreover, the reference mirror in the reference arm has been replaced by a blazed grating 36 such that the zeroth order is blocked by the opaque disc, the optical interference occurring between the 1st order (dominant via the blaze effect) diffracted by the grating and the light backscattered by the sample, which is only partially blocked by the opaque disc. The dimensions of the aperture diaphragm in the plane 14 are adjusted such that the dimensions of the image of the aperture diaphragm in the plane of the opaque disc 25 are smaller than those of the opaque disc 25; thus, all of the specular reflections issued from the interferometer are blocked. Such a system makes it possible to do without antireflection treatments on the windows of the elements for holding the samples, in particular when it is not possible to work with immersion microscope objectives.
Whatever the full-field OCT imaging system used, the sensitivity of the system is however limited by the amount of light coming from the sample that may be detected by the camera. Specifically, if R is the reflection coefficient of the splitting element 30 and T its transmission coefficient, and if it is assumed that the splitting element is optically lossless (R=1−T), the maximum amount of light coming from the sample that it is possible to collect is equal to R×T=T(1−T) and has an optimum for R=T=0.5, this corresponding to a maximum amount of light coming from the sample equal to 25% of the amount of light emitted by the source.
One possible way of collecting a higher light flux coming from the sample would be to increase the light intensity of the source, but this is not always possible in particular because of limitations on the photon budget of the sources used and because of the photosensitivity of certain samples. Another one way would be to duplicate the detection channel in order to collect both the flux reflected and the flux transmitted by the splitting element, but this would require two cameras.
The present description proposes an original architecture for a full-field OCT imaging system that allows, with respect to known prior-art systems, the amount of light coming from the sample that may be detected by the camera to be increased, without however increasing the light flux incident on the sample or the number of cameras. Such a full-field OCT imaging system is particularly suitable for low-cost systems for applications such as the acquisition of fingerprints and imaging of the skin.